Linear light polarizers, in general, owe their properties of selectively passing radiation vibrating along a given electromagnetic radiation vector, and absorbing electromagnetic radiation vibrating along a second given electromagnetic radiation vector, to the anisotropic character of the polarizer. Dichroic polarizers are absorptive, linear polarizers having a vectoral anisotropy in the absorption of incident light. The term “dichroism” as used herein refers to the property of differential absorption and transmission of the components of an incident beam of light depending on the direction of vibration of the components. Generally, a dichroic polarizer will transmit radiant energy along one electromagnetic vector and absorb energy along a perpendicular electromagnetic vector. A beam of incident light, on entering a dichroic polarizer, encounters two different absorption coefficients, one low and one high, so that the emergent light vibrates substantially in the direction of low absorption (high transmission).
Examples of synthetic dichroic polarizers are intrinsic polarizers, e.g., a polyvinylene-based polarizer such as a K-type polarizer. An intrinsic polarizer derives its dichroism from the light-absorbing properties of its matrix, rather than from the light-absorbing properties of dye additives, stains, or suspended crystalline material. Typically, intrinsic polarizers comprise a sheet or film of oriented poly(vinyl alcohol) having an oriented suspension of a dehydration product of polyvinyl alcohol, i.e., polyvinylene. Intrinsic polarizers of this kind are typically formed by heating the polymeric film in the presence of an acidic vapor dehydration catalyst, such as vapors of hydrochloric acid, to produce conjugated polyvinylenes and unidirectionally stretching the polymeric film prior to, subsequent to, or during the dehydration step to align the poly(vinyl alcohol) matrix. By orienting the poly(vinyl alcohol) matrix unidirectionally, the transition moments of the conjugated polyvinylenes or chromophores are also oriented, and the material becomes visibly dichroic. A second orientation step or extension step and a boration treatment may be employed after the dehydration step. An improved K-type polarizer is known as a KE-type polarizer and has improved stability under various conditions such as high temperature.